In a mobile communication system, the fading occurs due to signal paths. As a result, signals are subject to random carrier-wave fluctuation. For this reason, the adoption of synchronous detection is difficult in low speed digital communication. Conventionally, therefore, delay detection is used in such communication. In carrying out the delay detection, it is necessary to detect a symbol timing. For this purpose, a symbol timing detecting circuit includes an amplitude information detecting circuit and a narrow band-pass filter.
In this symbol timing detecting circuit, a carrier-wave signal modulated by .pi./4 shift QPSK is supplied to the amplitude information detecting circuit, in which a symbol timing signal component generated. The symbol timing signal component is supplied to the low narrow band-pass filter, from which a symbol timing signal is obtained. The detail of the symbol timing detecting circuit is explained in the Japanese Patent Kokai No. 3-188736, the published date of which is Aug. 16, 1992.
According to the conventional symbol timing detecting circuit, however, there is a disadvantage in that it is difficult to be included in an LSI, because it is structured by analog circuits. Further, there is a disadvantage in that the circuit adjustment is required in each system, because a sampling timing of a received signal is not coincident with a phase of an output signal supplied from the narrow band-pass filter. Still further, there is a disadvantage in that it is impossible to receive a burst signal which is particular to a TDMA signal, because a synchronous pull-in time is large under the condition that "Q" of the narrow band-pass filter is high.